The present embodiments relate to a beam guiding magnet for deflecting a beam of electrically charged particles along a particle path.
Curved beam guiding magnets are widely used in particle accelerator systems for deflecting and/or focusing a beam of charged particles, such as electrons or ions. The particles accelerated to high kinetic energies in such a particle accelerator system are used increasingly in medical treatment, such as cancer treatment. DE 199 04 675 A1 discloses a beam guiding magnet and an irradiation system with a beam guiding magnet. U.S. Pat. No. 4,870,287 discloses an irradiation system for medical treatment. The irradiation systems include a particle source and an accelerator for generating a high-energy particle beam. The high-energy particle beam is aimed at a region of a subject (treatment area), such as a growth, that is to be irradiated.
The region to be irradiated is scanned by the particle beam because the region is typically a spatially extended region. To obtain a scanning motion at the region to be irradiated, the particle beam is deflected out of its path by small angles. The deflection is compensated for by a deflection magnet in the beam direction. The beam strikes the site to be irradiated with a parallel offset.
The beam dose in the surrounding region, which is the region not to be treated, of the body of a subject should be minimal. To minimize the beam dose in the region not to be treated, the region to be treated is irradiated from different directions, so that the beam exposure will be distributed over the largest possible volume in the surrounding tissue. Depending on the location of the region to be irradiated in the body of the subject, the direction that the particle beam strikes the region to be irradiated may be selected such that the particle beam travels the shortest possible distance through the body of the subject to the region to be irradiated.
To irradiate a subject from different directions, the particle beam, along an axis predetermined by the accelerator, is shot (directed) into a gantry that is rotatable about the axis predetermined by the particle beam.
A gantry in this connection is an arrangement of various beam guiding magnets. The beam guiding magnets deflect the particle beam multiple times out of its original direction, so that after leaving the gantry, the beam strikes the area to be irradiated at a defined angle. Typically, the particle beam strikes the region to be irradiated at an angle of 45 to 90°, relative to the axis of rotation of the gantry.
The beam guiding magnets are located on a frame, which is part of the gantry, in such a way that the particle beam emerging from the gantry always extends through a fixed region to be irradiated, called the isocenter. The region to be treated can be irradiated from a plurality of sides. The beam dose in the region surrounding the isocenter can be distributed over a large volume, so that the beam exposure outside the isocenter can be relatively slight (small). If the gantry is not rotated during the irradiation, then the gantry can be set such that the beam takes the shortest possible route through the body of the patient on its way, for example, to the growth.
For irradiating a large growth or a large tumor, not only a variation of the angle at which the particle beam strikes the region to be irradiated, but a variation in the kinetic energy of the particles and a variation of the lateral site coordinates at the point where the particle beam strikes are desirable. For varying the lateral site coordinates of the particle beam, scanner magnets are typically integrated with the gantry. With the aid of these scanner magnets, the particle beam can be deflected by small angles each in a horizontal and a vertical plane. The deflections of the particle beam that are brought about by the scanner magnets typically have to be compensated for by the magnets that follow in the beam direction in such a way that the particle beam leaves the gantry in the form of virtually parallel beams.
Because of the aforementioned conditions placed on the magnets of a gantry, ion-optical demands are made in terms of the construction of the beam guiding magnets. Coil designs known from the prior art are generally optimized with respect to these criteria.
Such beam guiding magnets have a magnetic field that cannot be ignored in their outer space. The term “outer space of the beam guiding magnet” should be understood in this connection to mean the region that is not surrounded by the individual magnet coils of the beam guiding magnet.
The magnetic flux densities of a beam guiding magnet are typically between 20 mT and 50 mT in the region of the isocenter. These magnetic fields at the site of the isocenter are undesirable. For treating patients with pacemakers, a magnetic flux density of only 0.5 mT in the region of the patient (e.g., the patient's room) and in the region of the isocenter (e.g., the region of a tumor that may be present) is permitted.
Passive magnetic shielding of the patient's room is possible. However, a passive ferromagnetic shield has a high weight. A passive magnetic shield exhibits a nonlinear behavior with regard to the interaction with the electrically charged particle beam that is deflected by the beam guiding magnet.
Depending on the energy of the electrically charged particles of the particle beam that are deflected by the beam guiding magnet, the coils of the beam guiding magnet are typically subjected to currents, adapted to the particle energy, for deflecting the particle beam. Depending on the current supplied to the coils of the beam guiding magnet, these coils generate a varying magnetic field for deflecting the particle beam, and consequently they also generate a varying remote field. The remote field of the beam guiding magnet is kept away from the patient by passive magnetic shielding that may be present. In the material comprising the passive magnetic shielding, depending on the magnetic fields acting on it, corresponding electric currents are induced that lead to the buildup of contrary magnetic fields. If the magnetic fields originating at the beam guiding magnet or the coils of the beam guiding magnet vary, then the currents induced in the passive magnetic shielding vary as well.
To irradiate a patient inside a patient's room, the passive magnetic shielding must have an aperture for the beam of electrically charged particles to pass through. In the region of the aperture, the magnetic conditions vary where the currents induced in the passive magnetic shielding are varying. Each time a coil current of an individual coil of the deflection magnet varies, the magnetic conditions in the region of the aperture of the passive magnetic shielding vary. Each time the coil current of an individual coil of the deflection magnet varies, the beam of electrically charged particles may need to be readjusted.